Stents are well known medical devices that have been used for maintaining the patency of a large variety of vessels of the human body. The most frequent use for stents is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than twenty years, many stent designs still lack the required flexibility and radial rigidity to provide an optimum clinical result and also have fractures after many years of exposure to the flexing of a coronary artery.
Most current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed by connecting relatively straight segments with straight struts as the end crown (as with the Palmaz stent sold by Cordis a Johnson & Johnson Company) or with curved crowns (as with the BX Velocity stent, described in U.S. Pat. No. 6,190,403 by Fischell et al, sold by Cordis a Johnson & Johnson Company). In any case, each circumferential set of strut members forms a closed cylindrical ring that opens up as the stent expands and forms the cylindrical structure that acts as a scaffold which supports the dilated arterial wall. There are two basic forms of circumferential sets of strut members: full-slot circumferential sets of strut members having structures where the circumferential set of strut members open radial outward from a series of rectangles to form diamond like structures (the Palmaz stent) and half-slot circumferential sets of strut members which resemble more of a sine wave circumferential ring (such as the BX Velocity stent). The longitudinal dimension of the opening inside each circumferential set of strut members of the unexpanded stent is called the slot length.
In FIGS. 7-10 of U.S. Pat. No. 5,102,417, (the '417 patent) the Palmaz spiral stent design is shown that has full-slot circumferential sets of strut members with every straight end, square crown connected by a helical connector to a crown of the adjacent full-slot circumferential set of strut members. This design typically has only three full-slot circumferential sets of strut members for a stent that is 15 mm long. This full-slot design creates a long, straight, rigid, longitudinal structure with slot length typically greater than 4 mm in the unexpanded stent. This long slot length limits the stent's flexibility and therefore limits the stent's ability to be advanced through a curved coronary artery. Another disadvantage of the long slot design of Palmaz is that the long and rigid circumferential sets of strut members cause the ends of the circumferential sets of strut members to separate from the balloon and engage the arterial wall when the stent is advanced through a curved coronary artery. This phenomenon is known as “fish scaling” because the ends of the long strut members stick out from the surface of a curved balloon like the scale of a fish. Fish scaling can cause stent embolization or it can prevent the stent from being advanced through a highly curved coronary artery. The long, longitudinally straight structure of the Palmaz stent having the full-slot circumferential sets of strut members also have a tendency to easily slide off the delivery balloon thereby increasing the probability of stent embolization. In addition, with every crown of each full-slot circumferential set of strut members connected to the adjacent circumferential set of strut members, the ability of the stent to bend around a curve is limited. It is the lack of flexibility that prevented the stent described in the '417 patent from becoming a great commercial success. For the purposes of this disclosure, stents like that shown in FIGS. 7-10 of '417 patent, with each end crown of a circumferential set of strut members connected to a crown of an adjacent circumferential set of strut members is called a fully connected or closed cell stent. A stent with less than half of the crowns connected is generally called an open cell stent and a stent with every other crown connected is called a hybrid stent. It should also be noted that stent design of the '417 patent has struts that have a uniform width even though having a variable strut width can offer performance advantages as to maximizing the stent's radial rigidity while limiting maximum strain to a level below that that can cause fracture and fatigue failure.